Outline

One of the main benefits of radiotherapy with protons and heavy ions is the physical dose distribution characterised by the Bragg Peak and the ensuing sharp dose fall-off. However, these strong dose gradients also entail the need for an irradiation accuracy of approx. 1–2 mm. Real uncertainties can exceed this requirement by as much as an order of magnitude. Partially, this can be attributed to flaws in dose calculation and to the lack of knowledge about the quantitative relation of delivered dose to secondary radiation utilised by in vivo quality control. Nuclear interactions of the particle beam with human tissue represent key processes in these applications. The necessary accurate modelling of particle transport can only be achieved with Monte Carlo simulation. GEANT4 is a generic, object-oriented Monte Carlo toolkit developed by CERN. It provides comprehensive functionality while retaining high flexibility and potential for expansion. Albeit initially destined for High Energy Physics, GEANT4 is increasingly employed for particle therapy simulations which cover the low energy range.

In August 2007 a new project has been launched in a cooperation between the RWTH Aachen Physics Department, the University Hospital Aachen and the Philips Research Laboratories Aachen. The project aim is to validate GEANT4 nuclear interaction models for use in proton and ion therapy. For this purpose a dedicated detector system has been designed and implemented in GEANT4.

Methods: The detector is primarily destined for the identification of heavy fragments produced by proton and heavy ion beams in thin targets. The detection of the heavy target fragments and, thereby, the nuclear reaction channels is achieved through measurement of time-of-flight (TOF) and dE/dx. The primary beam hits the START detector and initialises the TOF measurement before it passes through two VETO detectors and impinges on the target. Successively, the secondary particles emanating from the target travel a short distance of 70 / 80 cm through vacuum (~1 mbar) before they hit one of the 20 STOP detectors.

Preliminary Results: GEANT4 simulations with a proton beam of 200 MeV show, so far, that the detection of heavy target fragments is generally possible. However, due to their low kinetic energy the heavy fragments are caught inside the STOP scintillators so that correlation curves overlap for particles with nearly equal mass.

Discussion and outlook: The first comparison of experimental data with the simulation will be possible after our next beam-time at COSY Juelich. Future simulations will show to what extent the current setup can be used for validation experiments with carbon ions. Thin scintillators may be employed in future in order to properly discriminate between heavy target fragments with nearly equal mass. According GEANT4 simulations are currently running. Also, target materials other than Graphite will be investigated at varying beam energies.